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Abstract:

In an embodiment, a bearing apparatus comprises a first bearing assembly
including a plurality of circumferentially-spaced first bearing elements
each of which includes a first bearing surface. The bearing apparatus
further includes a second bearing assembly including a plurality of
circumferentially-spaced second bearing elements each of which includes a
second bearing surface oriented to engage the first bearing surfaces of
the first bearing assembly during operation. At least one of the second
bearing elements may be circumferentially spaced from an adjacent one of
the second bearing elements by a lateral spacing greater than a lateral
dimension of the at least one of the second bearing elements.

Claims:

1. A bearing apparatus, comprising: a first bearing assembly including a
plurality of circumferentially-spaced first bearing elements each of
which includes a first bearing surface, each of the plurality of
circumferentially-spaced first bearing elements circumferentially spaced
from a circumferentially adjacent one of the plurality of
circumferentially-spaced first bearing elements by a respective first
lateral spacing less than a respective first lateral dimension of the
circumferentially adjacent one of the plurality of
circumferentially-spaced first bearing elements; and a second bearing
assembly including a plurality of circumferentially-spaced second bearing
elements each of which includes a second bearing surface oriented to
engage the first bearing surfaces of the first bearing assembly during
operation, each of the plurality of circumferentially-spaced second
bearing elements circumferentially spaced from a circumferentially
adjacent one of the plurality of circumferentially-spaced second bearing
elements by a respective second lateral spacing greater than a respective
second lateral dimension of the circumferentially adjacent one of the
plurality of circumferentially-spaced second bearing elements, the second
lateral spacing greater than the first lateral spacing.

2. The bearing apparatus of claim 1 wherein the respective second lateral
spacing is at least about two times an average lateral dimension of the
plurality of circumferentially-spaced second bearing elements.

3. The bearing apparatus of claim 1 wherein the plurality of
circumferentially-spaced second bearing elements are substantially
equally circumferentially spaced from each other.

4. The bearing apparatus of claim 1 wherein the respective first lateral
spacing is less than an average first lateral dimension exhibited by the
plurality of circumferentially-spaced first bearing elements.

5. The bearing apparatus of claim 1 wherein the respective second lateral
spacing is equal to or greater than the respective first lateral spacing.

6. The bearing apparatus of claim 1 wherein the second bearing assembly
comprises a bearing support including the plurality of
circumferentially-spaced second bearing elements mounted thereto, the
bearing support including at least one flow obstruction element
positioned and configured to provide a selected fluid flow over the
second bearing surfaces of the plurality of circumferentially-spaced
second bearing elements.

7. The bearing apparatus of claim 6 wherein the at least one flow
obstruction element is positioned circumferentially between
circumferentially adjacent second bearing elements of the plurality of
circumferentially-spaced second bearing elements and comprises a terminal
surface positioned below each of the second bearing surfaces.

8. The bearing apparatus of claim 6 wherein the at least one flow
obstruction element is laterally spaced a distance from a
circumferentially adjacent second bearing element of the plurality of
circumferentially-spaced second bearing elements less than an average
lateral dimension of the plurality of circumferentially-spaced second
bearing elements.

9. The bearing apparatus of claim 1 wherein at least some of the
plurality of circumferentially-spaced second bearing elements are more
thermally stable than the plurality of circumferentially-spaced first
bearing elements.

10. The bearing apparatus of claim 1 wherein the number of the second
bearing elements is less than the number of the first bearing elements.

11. The bearing apparatus of claim 1 wherein the first bearing assembly
comprises a first bearing support including the plurality of
circumferentially-spaced first bearing elements mounted thereto, and
wherein the second bearing assembly comprises a second bearing support
including the plurality of circumferentially-spaced second bearing
elements mounted thereto.

12. The bearing apparatus of claim 1 wherein the plurality of
circumferentially-spaced first bearing elements and the plurality of
circumferentially-spaced second bearing elements are distributed about a
thrust axis.

13. The bearing apparatus of claim 1 wherein the first bearing surfaces
of the plurality of circumferentially-spaced first bearing elements and
the second bearing surfaces of the plurality of circumferentially-spaced
second bearing elements are generally radially oriented.

14. The bearing apparatus of claim 1 wherein each of the first and second
bearing surfaces comprises polycrystalline diamond.

15. A bearing apparatus, comprising: a first bearing assembly including a
plurality of circumferentially-spaced first bearing elements each of
which includes a first bearing surface, each of the plurality of
circumferentially-spaced first bearing elements circumferentially spaced
from an immediately circumferentially successive one of the plurality of
circumferentially-spaced first bearing elements by a respective first
lateral spacing less than a respective first lateral dimension of the
immediately circumferentially successive one of the plurality of
circumferentially-spaced first bearing elements; and a second bearing
assembly including a plurality of circumferentially-spaced second bearing
elements each of which includes a second bearing surface oriented to
engage the first bearing surfaces of the first bearing assembly during
operation, each of the plurality of circumferentially-spaced second
bearing elements circumferentially spaced from an immediately
circumferentially successive one of the plurality of
circumferentially-spaced second bearing elements by a respective second
lateral spacing greater than a respective second lateral dimension of the
immediately circumferentially successive one of the plurality of
circumferentially-spaced second bearing elements, wherein the second
lateral spacing greater than the first lateral spacing, and wherein the
respective second lateral spacing is equal to or greater than the
respective first lateral spacing.

16. The bearing apparatus of claim 15 wherein the number of the second
bearing elements is less than the number of the first bearing elements,
and wherein at least some of the plurality of circumferentially-spaced
second bearing elements are more thermally stable than the plurality of
circumferentially-spaced first bearing elements.

17. The bearing apparatus of claim 15 wherein the respective second
lateral spacing is at least about two times an average lateral dimension
of the plurality of circumferentially-spaced second bearing elements.

18. The bearing apparatus of claim 15 wherein the plurality of
circumferentially-spaced second bearing elements are substantially
equally circumferentially spaced from each other.

19. The bearing apparatus of claim 15 wherein the respective first
lateral spacing is less than an average first lateral dimension exhibited
by the plurality of circumferentially-spaced first bearing elements.

20. The bearing apparatus of claim 15 wherein the respective second
lateral spacing is equal to or greater than the respective first lateral
spacing.

21. The bearing apparatus of claim 15 wherein the second bearing assembly
comprises a bearing support including the plurality of
circumferentially-spaced second bearing elements mounted thereto, the
bearing support including at least one flow obstruction element
positioned and configured to provide a selected fluid flow over the
second bearing surfaces of the plurality of circumferentially-spaced
second bearing elements.

22. The bearing apparatus of claim 15 wherein each of the first and
second bearing surfaces comprises polycrystalline diamond.

23. A motor assembly for use in a drilling system, the motor assembly
comprising: a motor operable to apply torque to a rotary drill bit, the
motor operably coupled to a bearing apparatus, the bearing apparatus
including: a first bearing assembly including a plurality of
circumferentially-spaced first bearing elements each of which includes a
first bearing surface, each of the plurality of circumferentially-spaced
first bearing elements circumferentially spaced from a circumferentially
adjacent one of the plurality of circumferentially-spaced first bearing
elements by a respective first lateral spacing less than a respective
first lateral dimension of the circumferentially adjacent one of the
plurality of circumferentially-spaced first bearing elements; and a
second bearing assembly including a plurality of circumferentially-spaced
second bearing elements each of which includes a second bearing surface
oriented to engage the first bearing surfaces of the first bearing
assembly during operation, each of the plurality of
circumferentially-spaced second bearing elements circumferentially spaced
from a circumferentially adjacent one of the plurality of
circumferentially-spaced second bearing elements by a respective second
lateral spacing greater than a respective second lateral dimension of the
circumferentially adjacent one of the plurality of
circumferentially-spaced second bearing elements, the second lateral
spacing greater than the first lateral spacing.

24. The motor assembly of claim 23 wherein the number of the second
bearing elements is less than the number of the first bearing elements,
and wherein at least some of the plurality of circumferentially-spaced
second bearing elements are more thermally stable than the plurality of
circumferentially-spaced first bearing elements.

25. The motor assembly of claim 23 wherein the respective second lateral
spacing is at least about two times an average lateral dimension of the
plurality of circumferentially-spaced second bearing elements.

26. The motor assembly of claim 23 wherein the plurality of
circumferentially-spaced second bearing elements are substantially
equally circumferentially spaced from each other.

27. The motor assembly of claim 23 wherein the respective first lateral
spacing is less than an average first lateral dimension exhibited by the
plurality of circumferentially-spaced first bearing elements.

28. The motor assembly of claim 23 wherein the respective second lateral
spacing is equal to or greater than the respective first lateral spacing.

29. The motor assembly of claim 23 wherein the second bearing assembly
comprises a bearing support including the plurality of
circumferentially-spaced second bearing elements mounted thereto, the
bearing support including at least one flow obstruction element
positioned and configured to provide a selected fluid flow over the
second bearing surfaces of the plurality of circumferentially-spaced
second bearing elements.

30. The motor assembly of claim 23 wherein each of the first and second
bearing surfaces comprises polycrystalline diamond.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.
12/394,489 filed on 27 Feb. 2009, the disclosure of which is
incorporated, in its entirety, by this reference.

BACKGROUND

[0002] Subterranean drilling systems that employ downhole drilling motors
are commonly used for drilling boreholes in the earth for oil and gas
exploration. FIG. 1 is a schematic isometric cutaway view of a prior art
subterranean drilling system 100. The subterranean drilling system 100
includes a housing 102 enclosing a downhole drilling motor 104 that is
operably connected to an output shaft 106. A thrust-bearing apparatus 108
is also operably coupled to the downhole drilling motor 104. A rotary
drill bit 112 configured to engage a subterranean formation and drill a
borehole is connected to the output shaft 106. The rotary drill bit 112
is shown as a roller-cone bit including a plurality of roller cones 114.
As the borehole is drilled, pipe sections may be connected to the
subterranean drilling system 100 to form a drill string capable of
progressively drilling the borehole to a greater depth within the earth.

[0003] The thrust-bearing apparatus 108 includes a stator 116 that does
not rotate and a rotor 118 that is attached to the output shaft 106 and
rotates with the output shaft 106. The stator 116 and rotor 118 each
include a plurality of bearing elements 120 that may be fabricated from
polycrystalline diamond compacts ("PDCs") that provide diamond bearing
surfaces that bear against each other during use.

[0004] In operation, high-pressure drilling fluid is circulated through
the drill string and power section (not shown) of the downhole drilling
motor 104, usually prior to the rotary drill bit 112 engaging the bottom
of the borehole, to generate torque and rotate the output shaft 106 and
the rotary drill bit 112 attached to the output shaft 106. When the
rotary drill bit 112 engages the bottom of the borehole, a thrust load is
generated, which is commonly referred to as "on-bottom thrust" that tends
to compress the thrust-bearing apparatus 108. The on-bottom thrust is
carried, at least in part, by the thrust-bearing apparatus 108. Fluid
flow through the power section may cause what is commonly referred to as
"off-bottom thrust," which is carried, at least in part, by another
thrust-bearing apparatus that is not shown in FIG. 1. The drilling fluid
used to generate the torque for rotating the rotary drill bit 112 exits
openings formed in the rotary drill bit 112 and returns to the surface,
carrying cuttings of the subterranean formation through an annular space
between the drilled borehole and the subterranean drilling system 100.
Typically, a portion of the drilling fluid is diverted by the downhole
drilling motor 104 to cool and lubricate the bearing elements 120 of the
thrust-bearing apparatus 108.

[0005] The off-bottom and on-bottom thrust carried by the thrust-bearing
apparatuses can be extremely large. The operational lifetime of the
thrust-bearing apparatuses often determines the useful life of the
subterranean drilling system 100. Therefore, manufacturers and users of
subterranean drilling systems continue to seek improved bearing
apparatuses.

SUMMARY

[0006] Embodiments of the invention are directed to bearing apparatuses
comprising a bearing assembly including bearing elements, with at least
one bearing element spaced from an adjacent bearing element by a lateral
spacing greater than a lateral dimension of the at least one bearing
element. The disclosed bearing apparatuses may be used in a number of
applications, such as downhole motors in subterranean drilling systems or
directional drilling systems, roller-cone drill bits, and many other
applications.

[0007] In an embodiment, a bearing apparatus comprises a first bearing
assembly including a plurality of circumferentially-spaced first bearing
elements each of which includes a first bearing surface. The bearing
apparatus further includes a second bearing assembly including a
plurality of circumferentially-spaced second bearing elements each of
which includes a second bearing surface oriented to engage the first
bearing surfaces of the first bearing assembly during operation. At least
one of the second bearing elements may be circumferentially spaced from
an adjacent one of the second bearing elements by a lateral spacing
greater than a lateral dimension of the at least one of the second
bearing elements.

[0008] Other embodiments include downhole motors for use in drilling
systems that may utilize any of the disclosed bearing apparatuses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The drawings illustrate several embodiments of the invention,
wherein identical reference numerals refer to identical elements or
features in different views or embodiments shown in the drawings.

[0010] FIG. 1 is a schematic isometric cutaway view of a prior art
subterranean drilling system including at least one thrust-bearing
apparatus.

[0011] FIG. 2A is an isometric cutaway view of a thrust-bearing apparatus
according to an embodiment of the invention.

[0012]FIG. 2B is an isometric view of the first bearing assembly shown in
FIG. 2A.

[0013]FIG. 2c is an isometric view of the second bearing assembly shown
in FIG. 2A.

[0014]FIG. 2D is a partial cross-sectional view of the second bearing
assembly shown in FIG. 2c that details a bearing element and a flow
obstruction element thereof.

[0015]FIG. 3A is an isometric view of a radial bearing apparatus
according to an embodiment of the invention.

[0016]FIG. 3B is an isometric view of the inner race shown in FIG. 3A.

[0017] FIG. 3C is an isometric view of the outer race shown in FIG. 3A.

[0018]FIG. 3D is a partial cross-sectional view of the outer race that
details a bearing element and a flow obstruction element thereof.

[0019]FIG. 4 is a side cross-sectional view of an embodiment of a bearing
element suitable for use in any of the bearing assemblies disclosed
herein.

[0020]FIG. 5 is a schematic isometric cutaway view of an embodiment of a
subterranean drilling system that includes at least one of the
thrust-bearing apparatuses shown in FIG. 2A.

DETAILED DESCRIPTION

[0021] Embodiments of the invention are directed to bearing apparatuses
comprising a bearing assembly including bearing elements, with at least
one bearing element spaced from an adjacent bearing element by a lateral
spacing greater than a lateral dimension of the at least one bearing
element. The disclosed bearing apparatuses may be used in a number of
applications, such as downhole motors in subterranean drilling systems or
directional drilling systems, roller-cone drill bits, and many other
applications.

[0022] FIG. 2A is an isometric cutaway view of a thrust-bearing apparatus
200, suitable for use in a subterranean drilling system, according to an
embodiment of the invention. The thrust-bearing apparatus 200 includes a
first bearing assembly 202 and a second bearing assembly 204. One of the
first bearing assembly 202 or the second bearing assembly 204 may serve
as a rotor and the other one of the first bearing assembly 202 or the
second bearing assembly 204 may serve as a stator in the thrust-bearing
apparatus 200. One or both of the first bearing assembly 202 and the
second bearing assembly 204 may rotate about a thrust axis 206 (FIG. 2B)
along which thrust may be generally directed during use.

[0023]FIG. 2B is an isometric view of the first bearing assembly 202
shown in FIG. 2A. The first bearing assembly 202 includes a first bearing
support ring 208 defining an aperture 210 through which a shaft of, for
example, a downhole drilling motor may pass. The first bearing support
ring 208 may comprise a metallic material (e.g., steel) or a more
wear-resistant material, such as cemented tungsten carbide, silicon
carbide, or another more wear-resistant material. The first bearing
support ring 208 includes a plurality of circumferentially-spaced first
bearing elements 212 mounted thereto and distributed about the thrust
axis 206 (FIG. 2B). For example, the first bearing elements 212 may be
mounted to the first bearing support ring 208 by brazing or
press-fitting, via one or more fasteners, or another suitable technique.
Each of the first bearing elements 212 includes a bearing surface 214.
The first bearing elements 212 exhibit an average first lateral dimension
(e.g., an average diameter) that may be determined by taking the average
of the respective maximum lateral dimensions of each first bearing
element 212. At least one first bearing element 212 may be separated from
an adjacent first bearing element 212 by a respective first lateral
spacing 218 that is less than a lateral dimension 216 (e.g., a diameter)
of the at least one first bearing element 212. The first lateral spacing
218 may be measured as a linear distance between adjacent first bearing
elements 212 or an arc length between adjacent first bearing elements 212
based on a reference circle that extends about the thrust axis 206. In an
embodiment, a portion of the first bearing elements 212 or each first
bearing element 212 may be separated from an adjacent first bearing
element 212 by a respective lateral spacing 218 that is less than the
average first lateral dimension of the first bearing elements 212.

[0024] In some embodiments, the first bearing elements 212 may be
substantially equally circumferentially spaced from each other, with the
respective first lateral spacing 218 between adjacent first bearing
elements 212 being approximately the same. However, in other embodiments,
the first bearing elements 212 may be non-uniformly circumferentially
spaced from each other.

[0025]FIG. 2c is an isometric view of the second bearing assembly 204
shown in FIG. 2A. The second bearing assembly 204 includes a second
bearing support ring 220 defining an aperture 222 through which the shaft
of, for example, the downhole drilling motor may pass. The second bearing
support ring 220 may be made from the same or similar materials as the
first bearing support ring 208 (FIGS. 2A and 2B). The second bearing
support ring 220 includes a plurality of circumferentially-spaced second
bearing elements 224 mounted thereto using any of the previously
mentioned mounting techniques. The second bearing elements 224 are
distributed about the thrust axis 206 (FIG. 2B). The number of second
bearing elements 224 may be substantially less than the number of first
bearing elements 212 (FIG. 2B). For example, the first bearing assembly
202 may include more than two to four times (e.g., three times) the
number of bearing elements included in the second bearing assembly 204.
As merely a non-limiting embodiment, the first bearing assembly 202 may
include nineteen of the first bearing elements 212 and the second bearing
assembly 204 may include six of the second bearing elements 224. Each
second bearing element 224 includes a bearing surface 226 that opposes
and bears against one or more of the bearing surfaces 214 (FIG. 2B)
during use.

[0026] Still referring to FIG. 2c, the second bearing elements 224 exhibit
an average second lateral dimension (e.g., an average diameter) that may
be determined by taking the average of the respective maximum lateral
dimensions of each second bearing element 224. At least one second
bearing element 224 may be separated from an adjacent second bearing
element 224 by a respective second lateral spacing 230 that is greater
than a second lateral dimension 228 (e.g., a diameter) of the at least
one second bearing element 224. The second lateral spacing 230 may be
measured as a linear distance between adjacent second bearing elements
224 or an arc length between adjacent second bearing elements 224 based
on a reference circle that extends about the thrust axis 206. In an
embodiment, a portion of the second bearing elements 224 or each second
bearing element 224 may be separated from an adjacent second bearing
element 224 by a respective lateral spacing 230 that is less than the
average second lateral dimension of the second bearing elements 224. In
an embodiment, the second lateral spacing 230 may be at least about two
times (e.g., about two to about four times) the average second lateral
dimension. The second lateral dimension 228 of each second bearing
element 224 may be equal to or greater than the first lateral spacing 218
(FIG. 2B) between adjacent first bearing elements 212 (FIG. 2B) to
prevent the first and second bearing assemblies 202 and 204 from
interlocking with each other during use.

[0027] In some embodiments, the second bearing elements 224 may be
substantially equally circumferentially spaced from each other, with the
respective second lateral spacing 230 between adjacent second bearing
elements 224 being approximately the same. However, in other embodiments,
the second bearing elements 224 may be non-uniformly circumferentially
spaced from each other.

[0028] During use, fluid (e.g., drilling mud) is pumped through a drill
string of a subterranean drilling system to effect rotation of a drill
bit (not shown). A portion of the fluid may also be permitted to flow
around and/or over the first bearing elements 212 and second bearing
elements 224 of the first and second bearing assemblies 202 and 204 for
cooling and/or lubrication thereof. Referring to the illustrated
embodiment shown in FIG. 2c and the partial cross-sectional view of FIG.
2D, in some embodiments, in order to enhance the flow rate around and
over the first bearing elements 212 (FIG. 2B) and the second bearing
elements 224, at least one flow obstruction element may be provided. For
example, a plurality of flow obstruction elements 232 may be provided,
with each flow obstruction element 232 positioned between adjacent second
bearing elements 224. Each flow obstruction element 232 may have a
lateral dimension 234 such that they occupy a major portion of the linear
or arcuate distance between the adjacent second bearing elements 224. For
example, in the illustrated embodiment, the flow obstruction elements 232
occupy the distance between adjacent bearing elements 224 such that a
minimum lateral dimension 236 of a gap between a flow obstruction element
232 and an adjacent second bearing element 224 is less than the average
second lateral dimension of the second bearing elements 224. In the
illustrated embodiment, the flow obstruction elements 232 have an arcuate
shape and may be integrally formed as part of the second bearing support
ring 220. However, the flow obstruction elements 232 may be removable,
replaceable, or may have other configurations that depart from the
illustrated configuration.

[0029] Referring specifically to FIG. 2D, the flow obstruction elements
232 may have a terminal surface 238 that is positioned below the bearing
surfaces 226 of the second bearing elements 224 by a distance 240. The
distance 240 may be chosen to be greater than the expected wear of the
second bearing elements 224 so that the terminal surfaces 238 (shown in
FIGS. 2C and 2D) of the flow obstruction elements 232 do not contact the
first bearing elements 212 during use. More particularly, each flow
obstruction element 232 may be configured so that fluid flow between
adjacent second bearing elements 224 may exhibit an average Reynolds
number of about 10,000 to about 60,000 (e.g., about 45,000 to about
60,000) during use. In an embodiment, the distance 240 may be about
0.0050 inches to about 0.030 inches and, more particularly, about 0.010
inches.

[0030] As an alternative to or in addition to the flow obstruction
elements 232 being employed on the second bearing assembly 204, in
another embodiment, flow obstruction elements may also be employed on the
first bearing assembly 202 between the first bearing elements 212
thereof.

[0031]FIG. 3A is an isometric view of a radial bearing apparatus 300,
suitable for use in a subterranean drilling system, according to an
embodiment of the invention. The radial bearing apparatus 300 includes an
inner race 302 received by an outer race 304. One of the inner race 302
or the outer race 304 may serve as a rotor and the other one of the inner
race 302 or the outer race 304 may serve as a stator in the radial
bearing apparatus 300. One or both of the inner race 302 and the outer
race 304 rotate about a rotation axis 306 during use.

[0032]FIG. 3B is an isometric view of the inner race 302 shown in FIG.
3A. The inner race 302 includes a first bearing support ring 308 defining
an aperture 310 through which a shaft or a spindle may be inserted. The
first bearing support ring 308 may be made from the same or similar
materials as the first bearing support ring 208 (FIG. 2A and 2B). The
first bearing support ring 308 includes a plurality of
circumferentially-spaced first bearing elements 312 mounted thereto using
any of the previously mentioned mounting techniques. The first bearing
elements 312 are distributed about the rotation axis 306. Each first
bearing element 312 includes a convexly-curved bearing surface 314
oriented in a radially outward direction. The first bearing elements 312
exhibit an average first lateral dimension that may be determined by
taking the average of the respective maximum lateral dimensions of each
first bearing element 312. At least one first bearing element 312 may be
separated from an adjacent first bearing element 312 by a respective
lateral spacing 318 that is less than a first lateral dimension (e.g., a
diameter) 316 of the at least one first bearing element 312 and may be
measured as an arc length between adjacent first bearing elements 312
based on a reference circle that extends about the rotation axis 306. In
an embodiment, a portion of or each first bearing element 312 may be
separated from an adjacent first bearing element 312 by a respective
lateral spacing 318 that is less than the average first lateral dimension
of the first bearing elements 312. The first bearing elements 312 may be
substantially equally circumferentially spaced or non-uniformly spaced
about the rotation axis 306.

[0033] FIG. 3C is an isometric view of the outer race 304 shown in FIG.
3A. The outer race 304 includes a second bearing support ring 320 having
a plurality of circumferentially-spaced second bearing elements 322
mounted thereto using any of the previously mentioned mounting
techniques. The second bearing support ring 320 may be made from the same
or similar materials as the first bearing support ring 208 (FIGS. 2A and
2B). The second bearing elements 322 are distributed about the rotation
axis 306. Each second bearing element 322 includes a concavely-curved
bearing surface 324 that corresponds to the curvature of the
convexly-curved bearing surfaces 314 of the first bearing elements 312
and is oriented in a radially inward direction. The second bearing
elements 322 exhibit an average lateral dimension that may be determined
by taking the average of the respective maximum lateral dimensions of
each second bearing element 322.

[0034] Still referring to FIG. 3C, at least one second bearing element 322
may be separated from an adjacent second bearing element 322 by a
respective lateral spacing 328 that is greater than a second lateral
dimension 326 of the at least one second bearing element 322 and may be
measured as an arc length based on a reference circle that extends about
the rotation axis 306. In an embodiment, a portion of or each second
bearing element 322 may be separated from an adjacent second bearing
element 322 by a respective lateral spacing 328 that is greater than the
average second lateral dimension of the second bearing elements 322. In
an embodiment, the lateral spacing 328 may be at least about two times
(e.g., about two to about four times) the average lateral dimension of
the second bearing elements 322. The average lateral dimension of the
second bearing elements 322 may be equal to or greater than the lateral
spacing 318 between adjacent first bearing elements 312 to prevent
interlocking of the inner race 302 and the outer race 304 during use. The
number of second bearing elements 322 may be substantially less than the
number of first bearing elements 312. For example, the first bearing
assembly 302 may include more than two to four times (e.g., three times)
the number of bearing elements included in the second bearing assembly
304. As merely a non-limiting embodiment, the inner race 302 may include
nineteen of the first bearing elements 312 and the outer race 304 may
include six of the second bearing elements 322.

[0035] In an embodiment, the second bearing elements 322 are substantially
equally circumferentially spaced about the rotation axis 306. However, in
other embodiments, the second bearing elements 322 may be
circumferentially non-uniformly spaced about the rotation axis 306.

[0036] During use, the bearing surfaces 314 of the first bearing elements
312 slidingly engage bearing surfaces 324 of the second bearing elements
322 as the inner race 302 rotates relative to the outer race 304.

[0037] During operation, fluid (e.g., drilling mud) may be pumped between
the inner race 302 and the outer race 304 to flow around and/or over the
first bearing elements 312 and second bearing elements 322 for cooling
and/or lubrication thereof. Referring to the illustrated embodiment shown
in FIG. 3C and the partial cross-sectional view of FIG. 3D, in some
embodiments, in order to provide a selected flow rate around and/or over
the first bearing elements 312 and the second bearing elements 322, a
plurality of flow obstruction elements 330 may be provided. Each flow
obstruction element 330 is positioned between adjacent second bearing
elements 322. Each flow obstruction element 330 may exhibit a maximum
lateral dimension or width 332 such that it occupies a major portion of
the arcuate distance between adjacent second bearing elements 322. For
example, in the illustrated embodiment, the flow obstruction elements 330
occupy the distance between adjacent bearing elements 322 such that an
angular width 334 of a gap between a flow obstruction element 330 and an
adjacent second bearing element 322 is less than the average lateral
dimension of the second bearing elements 322.

[0038] As an alternative to or in addition to the flow obstruction
elements 330 being employed on the second bearing assembly 304, in
another embodiment, flow obstruction elements may also be employed on the
first bearing assembly 302 between the first bearing elements 312
thereof.

[0039] Still referring to FIG. 3D, each flow obstruction element 330 may
include a terminal surface 338 (shown in FIGS. 3C and 3D as curved) that
is positioned below the bearing surfaces 324 of the second bearing
elements 322 by a distance 340. The distance 340 may be chosen to be
greater than the expected wear of the second bearing elements 322 so that
the terminal surfaces 338 of the flow obstruction elements 330 do not
contact the first bearing elements 312 during use. In an embodiment, the
distance 340 may be between 0.0050 inches and 0.030 inches and, more
particularly, about 0.010 inches.

[0040] The radial bearing apparatus 300 may be employed in a variety of
mechanical applications. For example, a roller-cone rotary drill bit may
employ the radial bearing apparatus 300. More specifically, the inner
race 302 may be mounted to a spindle of a roller cone and the outer race
304 may be affixed to an inner bore formed within the roller cone, and
the outer race 304 and inner race 302 may be assembled to form the radial
bearing apparatus 300. The radial bearing apparatus 300 may also be
employed in a downhole drilling motor and turbine.

[0041] Referring to FIG. 4, a number of different types of bearing
elements may be employed in the thrust-bearing apparatus 200 and radial
bearing apparatus 300. FIG. 4 is a side cross-sectional view of an
embodiment of a bearing element 400 suitable for use in any of the
bearing assemblies disclosed herein. The bearing element 400 may be a
super-hard compact that includes a super-hard table 402 of super-hard
material bonded to a substrate 404. The super-hard table 402 includes a
suitably configured bearing surface 406. For example, the bearing element
may be PDC including a polycrystalline diamond table bonded to a
cobalt-cemented tungsten carbide substrate.

[0042] The term "super-hard," as used herein, means a material having a
hardness at least equal to a hardness of tungsten carbide. The super-hard
table 402 may comprise any suitable super-hard material, such as silicon
carbide, a diamond-silicon carbide composite, polycrystalline cubic boron
nitride, polycrystalline cubic boron nitride and polycrystalline diamond,
or any other suitable super-hard material or combination of super-hard
materials.

[0043] As noted hereinabove, there may be fewer second bearing elements
224, 322 in the bearing assembly 204 and outer race 304 than there are
first bearing elements 212, 312 in the first bearing assembly 202 and
inner race 302. In some embodiments, a portion of or all of the second
bearing elements 224, 322 may be thermally-stable PDCs, while the first
bearing elements 212, 312 may be PDCs that are not as thermally stable
(e.g., a PDC in which a polycrystalline diamond table thereof has not
been leached of a metallic catalyst used to catalyze the formation of the
polycrystalline diamond). Utilizing thermally-stable PDCs for the second
bearing elements 224, 322 may compensate for their reduced load-bearing
surface area due to their reduced number compared to the first bearing
elements 212, 312.

[0044] A number of different types of thermally-stable PDCs may be used.
In an embodiment, a thermally-stable PDC may include a cemented carbide
substrate bonded to a polycrystalline diamond table. A portion of or
substantially all of the metallic catalyst used to catalyze formation of
the polycrystalline diamond table may be leached therefrom. Another
suitable thermally-stable PDC includes an at least partially leached
polycrystalline diamond table that is bonded to a cemented carbide
substrate. Yet another suitable thermally-stable PDC includes a
polycrystalline diamond table bonded to a cemented carbide substrate,
with interstitial regions between bonded diamond grains of the
polycrystalline diamond table having a nonmetallic catalyst disposed
therein (e.g., one or more alkali metal carbonates, one or more alkaline
metal carbonates, one or more alkaline earth metal hydroxides, or
combinations thereof), silicon carbide, or combinations of the foregoing.
As yet a further example, pre-sintered PCD tables may be bonded to a
substrate (or employed separately) in various configurations, such as
back-filled, leached, etc.

[0045] The thermal stability of a PDC may be evaluated by measuring the
distance cut in a granite workpiece prior to failure without using
coolant in a vertical turret lathe ("VTL") test. The distance cut is
considered representative of the thermal stability of the PDC. In some
embodiments, the second bearing elements 224, 322 may have a VTL-test
distance to failure, that is at least three times and, more particularly,
about five times greater than that of the first bearing elements 212,
312.

[0046]FIG. 5 is a schematic isometric cutaway view of a subterranean
drilling system 500 that includes at least one of the thrust-bearing
apparatuses 200 shown in FIG. 2A according to another embodiment. The
subterranean drilling system 500 includes a housing 502 enclosing a
downhole drilling motor 504 (i.e., a motor, turbine, or any other device
capable of rotating an output shaft) that is operably connected to an
output shaft 506. A first thrust-bearing apparatus 2001 (FIG. 2A) is
operably coupled to the downhole drilling motor 504 to form a motor
assembly. A second thrust-bearing apparatus 2002 (FIG. 2A) is
operably coupled to the output shaft 506. A rotary drill bit 508
configured to engage a subterranean formation and drill a borehole is
connected to the output shaft 506. The rotary drill bit 508 is shown as a
roller-cone bit including a plurality of roller cones 510. However, other
embodiments may utilize different types of rotary drill bits, such as a
fixed-cutter drill bit. As the borehole is drilled, pipe sections may be
connected to the subterranean drilling system 500 to form a drill string
capable of progressively drilling the borehole to a greater depth within
the earth.

[0047] The first thrust-bearing apparatus 2001 includes a first
bearing assembly 202 configured as a stator that does not rotate and a
second bearing assembly 204 configured as a rotor that is attached to the
output shaft 506 and rotates with the output shaft 506. The on-bottom
thrust generated when the drill bit 508 engages the bottom of the
borehole may be carried, at least in part, by the first thrust-bearing
apparatus 2001. The second thrust-bearing apparatus 2002
includes a first bearing assembly 202 configured as a stator that does
not rotate and a second bearing assembly 204 configured as a rotor that
is attached to the output shaft 506 and rotates with the output shaft
506. Fluid flow through the power section of the downhole drilling motor
504 may cause what is commonly referred to as "off-bottom thrust," which
may be carried, at least in part, by the second thrust-bearing apparatus
2002.

[0048] During use, drilling fluid may be circulated through the downhole
drilling motor 504 to generate torque and effect rotation of the output
shaft 506, and the second bearing assemblies 204 (i.e., the rotors) and
the rotary drill bit 508 attached thereto so that a borehole may be
drilled. A portion of the drilling fluid may also be used to lubricate
opposing bearing surfaces of the bearing elements of the thrust-bearing
apparatuses 2001 and 2002 of which only bearing elements 212
are illustrated in FIG. 5.

[0049] While various aspects and embodiments have been disclosed herein,
other aspects and embodiments are contemplated. The various aspects and
embodiments disclosed herein are for purposes of illustration and are not
intended to be limiting. Additionally, the words "including," "having,"
and variants thereof (e.g., "includes" and "has") as used herein,
including the claims, shall have the same meaning as the word
"comprising" and variants thereof (e.g., "comprise" and "comprises").